1. Field of the Invention
The present invention relates to a lateral electric field type liquid crystal display device and, more specifically, to a lateral electric field type liquid crystal display device with high luminance and wide viewing angles.
2. Description of the Related Art
Regarding the liquid crystal display devices especially in these days, widely employed is an IPS (In-Plane Switching) mode with which axes of liquid crystal molecules are rotated within a plane in parallel to the substrate by a lateral electric field. The IPS mode has almost no visual angle dependency for the angle of the rise of the molecule axes so that there is such an advantage that wide viewing angles can be acquired. Therefore, it is used broadly in large-screen television receivers, mobile phones, tablet terminals, and the like.
(Existing Technique 1)
FIG. 11 shows explanatory charts showing an existing liquid crystal display device 1010 with the IPS mode, which is also depicted in JP Patent No. 4603560 (Patent Document 1 (p. 8, FIG. 1)). The liquid crystal display device 1010 includes a liquid crystal display panel 1011, and pixels 1100 are arranged in matrix on the liquid crystal display panel 1011. FIG. 11A shows the external appearance of the liquid crystal display device 1010, FIG. 11B is an enlarged plan view of the pixel 1100, and FIG. 11C shows a sectional view taken along a line A-A′ of FIG. 11B, respectively.
In the pixel 1100, a scan signal wiring 1101 constituted with a first metal layer and two parallel common signal wirings 1102 are formed on a first glass substrate 1118. A first insulating film 1103 is formed on the scan signal wiring 1101 and the common signal wirings 1102, and a video signal wiring 1104 constituted with a second metal layer, a thin film transistor 1105, and a source electrode 1106 are formed on the first insulating film 1103.
A second insulating film 1107 made with an inorganic film is formed on the video signal wiring 1104, the thin film transistor 1105, and the source electrode 1106. Further, a third insulating film 1108 made with an organic film is formed on the second insulating film 1107.
Further, a comb-like pixel comb electrode 1109 and a comb-like common comb electrode 1110 made with a transparent conductive film are formed on the third insulating film 1108. Furthermore, it is also possible to employ a structure in which the pixel comb electrode 1109 and the common comb electrode 1110 are formed on the second insulating film 1107 without using the third insulating film 1108.
The video signal wiring 1104 is completely covered by the common electrode 1110 in the wiring width direction via the second insulating film 1107 and the third insulating film 1108. The pixel electrode 1109 and the common electrode 1110 are electrically connected to the source electrode 1106 and the common signal wiring 1102 via contact holes 1111 and 1112, respectively. The region where the common signal wiring 1102 and the source electrode 1106 overlap with each other becomes a storage capacitance.
Further, since the comb-like pixel electrode 1109 and the comb-like common electrode 1110 are both made with a transparent conductive film, the regions on the electrode also contribute to the transmittance. Employed herein is the structure in which the video signal wiring 1104 is completely covered by the common electrode 1110, so that it is possible to widen the aperture part to the vicinity of the video signal wiring 1104.
As a second substrate, a light shielding layer 1117, a color layer 1116 (R, G, B), and an overcoat layer 1115 are formed on a second glass substrate 1119 in this order. It is unnecessary to form a color layer for the case of monochrome display.
Alignment layers 1113 and 1114 are applied and calcined on the first substrate and the second substrate, respectively. After performing rubbing processing thereon in a prescribed direction, the first substrate and the second substrate are superimposed on one another, and a liquid crystal layer 1122 is sandwiched therebetween with a prescribed gap provided by a spacer. Polarization plates 1120 and 1121 are laminated on the outer side of the first substrate and the second substrate, respectively.
As other related techniques, there are following documents. Japanese Unexamined Patent Publication 2012-022344 (Patent Document 2) and Japanese Unexamined Patent Publication 2008-191669 (Patent Document 3) both disclose a liquid crystal display device which employs an FFS (Fringe-Field Switching) mode in a part of the IPS mode. Details thereof will be described in the latter section of “the problem to be solved by the present invention”.
With the technique of Patent Document 1 described above, it is possible to acquire a liquid crystal display device with high luminance by setting the aperture ratio relatively high while acquiring wide viewing angles by the IPS mode.
However, with this technique, the storage capacitance is formed by the overlapped part of the common signal wiring and the source electrode. Thus, it is required to form the common signal wiring in parallel to the scan signal wiring. Further, it is required to secure the storage capacitance also for the source electrode, so that the area cannot be reduced. Therefore, the aperture ratio in the vertical direction (i.e., luminance) cannot be improved.
In the meantime, a structure of the liquid crystal display device in which one out of the pixel electrode and the common electrode is formed as a plan-shape electrode and the other is formed as a linear-shape electrode, and the linear-shape electrode is disposed on the plan-shape electrode via an insulating film is referred to as an FFS (Fringe-Field Switching) mode.
With this mode, there is necessarily the region where the pixel electrode and the common electrode overlap with each other. Thus, the storage capacitance can be secured by using that part, so that the aperture part can be secured widely. However, the vertical component of the electric field is stronger in the FFS mode compared to that of the IPS mode, so that rise of the liquid crystal molecules becomes greater. Thus, it is inferior in terms of the viewing angles.
Therefore, it has been tried in some cases in the past to acquire a liquid crystal display device which exhibits the merits of each of the modes in terms of the viewing angles and the aperture ratio by combining the structures of the both of the IPS mode and the FFS mode.
(Existing Technique 2)
In Patent Document 2 (p. 20, FIG. 2), depicted is a transflective liquid crystal display device in which the transmissive region of the pixel is the IPS mode and the reflective region thereof is the FFS mode. This structure is targeted to improve the display quality of the reflective region but not targeted to improve the aperture ratio in the vertical direction.
Further, in the liquid crystal display device depicted in Patent Document 2, the entire reflective part is the FFS mode and the proportion of the area thereof is close to ½ of the entire display unit. Furthermore, the pixel electrode is made with two kinds of different materials, i.e., the reflective part is made with a metal layer and the transmissive part is made with a transparent conductive layer. Thus, a connection part is required between the both. Therefore, it is not possible to increase the light use efficiency by improving the aperture ratio and to acquire a liquid crystal display device of high luminance.
(Existing Technique 3)
In Patent Document 3 (p. 13, FIG. 1A), depicted is a transmissive type liquid crystal display device in which the both ends on the left and right sides of the pixel are in the IPS mode and the center part thereof is in the FFS mode.
The FFS mode is dominant in this structure, so that it is not possible to make the best use of the advantage of the viewing angle property of the IPS mode. Further, since the IPS mode part and the FFS mode part are divided along the direction of the FFS linear electrode, the IPS mode part and the FFS mode part cannot be optimized individually. Therefore, the advantages of each of the modes cannot be fully exhibited.
It is therefore an exemplary object of the present invention to provide a lateral electric field type liquid crystal display device which can achieve both the wide viewing angles and the high luminance by effectively combining the structures of the both modes called the IPS mode and the FFS mode.